Recently, non-aqueous electrolyte secondary batteries used as the main power source for mobile communications appliances and portable electronic appliances have high electromotive force and high energy density. The positive electrode of non-aqueous electrolyte secondary batteries usually comprises a lithium transition metal composite oxide as a positive electrode active material. Among lithium transition metal composite oxides, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and the like are preferable. These lithium transition metal composite oxides have a potential of 4 V or more relative to lithium.
In the case of non-aqueous electrolyte secondary batteries utilizing lithium ions (lithium ion secondary batteries), if the end of charge voltage of the battery is heightened, the capacity is increased commensurately. Hence, heightening the operating voltage of non-aqueous electrolyte secondary batteries is examined.
For example, in non-aqueous electrolyte secondary batteries including a manganese-containing spinel lithium oxide as a positive electrode active material, there has been a proposal to set the upper limit charge voltage in the range of 4.0 V to 4.5 V. Spinel lithium oxides are stable even at high potential (see Japanese Laid-Open Patent Publication No. 2001-307781).
Predominant non-aqueous electrolyte secondary batteries including a lithium cobalt oxide as a positive electrode active material have a high capacity and excellent characteristics such as cycle characteristics and storage characteristics. However, if such non-aqueous electrolyte secondary batteries are repeatedly charged up to high voltage and discharged, their capacity and the thermal stability of the active material degrade. Thus, the conventional end of charge voltage in normal operation is 4.2 V at most, and even if control circuit errors are allowed for, it is less than 4.25 V at most. If non-aqueous electrolyte secondary batteries are operated at a voltage of 4.25 V or higher, their safety may be particularly impaired.
Even in the case of the end of charge voltage being set to 4.2 V, if the battery is overcharged, for example, accidentally, the battery voltage increases to more than that. In such cases, it is also desired that the positive electrode active material maintain its stable crystal structure. Thus, there has been proposed a technique by which a specific element is incorporated in the form of solid solution in a composite oxide constituting the positive electrode active material (see Japanese Laid-Open Patent Publication No. 2002-203553).
Further, there has also been a proposal to use a mixture of specific two kinds of composite oxides as a positive electrode active material, in order to improve the thermal stability of the battery upon overcharge (see Japanese Laid-Open Patent Publication No. 2002-319398).